Another version of how not to build a model engine

[NickG]

S, now it is controlled by cam you can put a stronger spring to avoid any bouncing. I don't think it would though, as soon as it gets to its compression stroke it would shut and stay shut until there is an underpressure in cylinder.

Nick

[sorveltaja]

Repaired camshaft:


Stronger spring and tappet added to inlet valve. Engine is now ready for testing:

[sorveltaja]

Testings with new inlet cam didn't make the engine run. At first, the cam was rather egg-shaped, having too long opening/closing time, so I filed the cam a bit, to make the valve opening and closing more rapid:


After that, engine fires, and also runs at fast speed again, although I have tested it only with 1.9mm carb this far.

Addition: I tested the engine with 0.8mm carb also, and got similar results, as with spring-loaded intake cam. Engine fires, and almost runs.

[sorveltaja]

Current status of the engine. Hopefully it's noticeable on the video, how the engine responds to carb needle:

Previous sparking plug started to push out its ptfe insulation, causing severe running problems, so I had to make new, bit modded version, that should keep the insulator in place much better.

Test setup on the video:
Fuel: regular 95 gasoline with small amount of motor oil
Carburetor is the older one 1.9mm throat with 1.5mm hole ptfe plug in its butt its throat.
Ignition timing was about tdc(top dead center). I have tested both advancing, and retarding the timing though.

Generally, the engine has began to show more responsibility to the carb, and also it starts easier.

Maybe it just needs plenty of short running-in sessions.

[bogstandard]

S,

It has only just clicked with me , you are not using a throttle on the carb, so all you will ever get is high speed running, which is correct. When you adjust the needle, all you are doing is making the mixture more or less efficient, either weak, correct or rich. Just like a normal model free flight or control line engine.

What you really need is a throttling carburettor to give you speed control, which is a little more difficult than what you have now, but achievable.


John

[NickG]

Seeing that video it's obvious now! I have been studying Jan's engines for a couple of years now and I am so used to that petrol vapour carb that I didn't notice lack of throttle body either. Jan effectively changes the mixture to alter the speed but he is also choking the air at the same time so it's a bit like a throttle? (please correct me if I'm wrong). S , as John said you are purely changing the mixture, the throttle is full bore all the time.

Looks to be running really nicely by the way.

Nick

[chuck foster]

sorveltaja : that's one sweat running little engine but as others have said you need a carb that has a throttle.
i might have some plan's to build a little carb that might just do the trick for you.
i will dig them out and email them to you if you like.

chuck 

[NickG]

Not sure what model ones are like but I know from working with car SU carburettors they effectively have a cam on the throttle butterfly rod that pushes the needle down as you need to richen the mixture as you open the throttle. Not sure how fixed jet carbs work, number of jets that become uncovered by a rotating disc?

Nick

[Darren]

You could try a washer, or plate with a small hole to restrict the airflow. Expect to have to adjust the needle as well though.

Get it going and place the washer over the intake to see if it slows down. Different sized holes should give you different speeds. Worth having a go before making something.

[NickG]

Good point Darren. 

[sorveltaja]

Thanks for replies and suggestions .

Today I was testing the engine with different carb settings, and suddenly it just refused to start or run. So I double-checked every possible things.

Fuel system was ok, ignition system was ok, but the new spark plug started causing shortcuts, no matter what. Yesterday it worked just fine .

As gasoline itself doesn't conduct electricity much, in this case it was enough to cause almost electrochemical pair between spark plug and its core. Resulting black goo between them. With previous plug, that never happened.

It must have something to do with new spark plugs body material. It is made of an ordinary M6 screw, as was the previous one. Apparently they aren't same material.

Old and new plug:

Two cores, left was made of an M2 screw, right is stainless steel. Both was tested with new plug, but no difference.

The battery-led combination is a poor man's continuity tester. The 9V battery in picture is old, and it has low enough voltage, so it doesn't burn the led.   

[sorveltaja]

Old plug is on the left, new on the right:


Right one might have too little sparking surface, when compared to the left one. That also could be one thing, that causes the shortcut phenomena.

edit: I remember seeing great spark plug build tutorial on HMEM, and there it was: http://www.homemodelenginemachinist.com/index.php?topic=5653.0

When making the next spark plug, I think I'll follow gbritnell's advices .

[sorveltaja]

New spark plug body, made of stainless steel, except the nut, that was silver soldered:


Finished work:


Anyways, now it seems that the engine testing can continue  .

[Andy]

Sorveltaja, I've been following this with interest, and I admire your persistence. 

I'm afraid I can't offer any advice, but hope you get it going to your liking very soon!

[sorveltaja]

Andy, thanks for your reply .

I did running tests with new plug, and it seems to work ok. But the engine still has quite a reckless behaviour.

I have read the previous replies, and there is good suggestions about that carburetor thing.

Hopefully I finally can concentrate to test different carb variations, besides stomping bugs. One by one, if needed  .

[sorveltaja]

I made ptfe washer for the spark plug, and it seems to 'live', so that it's required to tighten the plug occasionally.

That's what I did. Until the exhaust valve hit the sparking plug, and got stuck .

Anyway, I got the exhaust valve to work, after some persuasion. Not sure how well it seals, but before starting to make a new valve, I'll test if it's possible to get the engine running with current valve.

[DICKEYBIRD]

I just finished catching up on this thread.  Great job and a wonderful example to all of us to persevere with our projects!

One thing to try is an exhaust throttle.  I have had great success with my small (Cox .010 to .049 c.i.) model airplane engines using exhaust throttles and fixed venturis.  Granted, these are 2-stroke engines and yours is a 4-stroke but it's certainly worth a try.  You might be surprised.

Very small throttleable carburetors are difficult to make work properly due to scaling issues.  Reynolds numbers and all that rot.

[sorveltaja]

Luckily I opened the cylinder head, as whole combustion chamber and valves were covered with black goo. No wonder, that the engine refused to run.

Maybe I used too much oil in the fuel. It's a run-in season for the engine anyway, so I thought, that more is better, than too little . Apparently this isn't glow engine after all, but fire breathing plain gasoline one.

That led me to discover another problem with recently made spark plugs. 

Distance between center electrode and plug body was too short, making it prone to conduct between them, or worst, cause a shortcut.

So I made a new spark plug, after seeing some commercial plug cutaways:

[sorveltaja]

Dickeybird, good point. I have heard about that exhaust throttle thing... but as far as I can remember, it's only good for two-strokers, as their performance might even advance, when suitable, counter-pressure creating tuned exhaust pipe is used.

Back to the project. I've made some running tests, after the recent exhaust valve hit spark plug -thing. The engine barely runs. Exhaust valve doesn't seat well enough. I'm surprised, that it isn't bent at all .

The engine is now disassembled, and I'm going to re-lap both valves (may be the 30th time, but who counts, as it's test engine ) . 

[sorveltaja]

The current crankcase cover is made of 1mm brass, and it's too thin, as it holds the breaker points assy also. When in use, it bends just a bit, letting the crankcase oil to leak out.

To fix that, I'll make a new one using 1.5mm mild steel:

[sorveltaja]

New crankcase cover in place, after some cleaning:


While inspecting other parts, I noticed, that both piston top and brass button had clear mark on them, caused by knocking on each other. Not good.
That should not have been possible . Anyways, I lowered the brass button about 1mm or so, to make sure, that it and piston doesn't get a chance to knock anymore: 

[sorveltaja]

So far, but not so good; when assembling the engine, piston didn't fit to the cylinder anymore, as it did before.

Cylinder liner is bent on its lower part, being more oval shaped, than round. I guess, that something went wrong, when I did the recent sealing/tightening with it and the cylinder head. Cylinder liner has only 0.5mm 0.2mm(~0.0078")wall thickness, so no wonder, that it finally gave up:


Just another unexpected weak spot in my design .

Not sure, how long it takes, but I'll have to make a new cylinder liner and piston, as current ones are now useless.

During this project, I've somehow began to get used to make new parts, as needed.

Cylinder/piston fitting is one of the most challenging thing to do on any engine, but then again, I consider it as a good chance to practice my skills.

I'll post the process pics as soon as I get a chance.

[sorveltaja]

At this point, cylinder lapping is at the stage, where the surface begins to be satin smooth, and almost even:


Bore diameter will be 0.5mm smaller, than on previous cylinder, thus allowing the old piston to be re-machined, and fitted.

[sorveltaja]

Todays effort:


New brackets are made of 1.5mm(~0.059") mild steel, to add more rigidity, while previous ones were made of 1mm(~0.039") brass.

Next thing to do is to drill screw holes to both bracket, and file the excess material off.

[sorveltaja]

As can be seen, once a perfectly working cylinder has now became two sacrificial drilling templates:


New brackets were super-glued in place, before drilling. Screws on the above pic are there securing, just in case, if that gluing gives up.

So it's time to start the filing process.